1. Field of the Invention
The present invention relates to an ink jet ink, an ink cartridge containing the ink jet ink, and an ink jet recording method.
2. Description of the Related Art
With improvements in image quality and recording speed, ink jet recording apparatuses are increasingly being used in business. Examples of performance requirements required for ink jet inks for use in business include, but are not limited to, ink reliability (such as ejection stability) and image fastness property (such as scratch resistance and highlighter resistance). In order to improve these performance requirements, various inks containing a polyurethane polymer and a pigment have been investigated (Japanese Patent Laid-Open No. 2008-266595 and Japanese Patent Laid-Open No. 2000-248213). Japanese Patent Laid-Open No. 2008-266595 discloses a pigment dispersion containing a polyurethane polymer that is produced by the reaction of a urethane prepolymer, a polyol, and a polyisocyanate. The urethane prepolymer has one or two carboxy groups per molecule and has a defined isocyanate reaction rate. Japanese Patent Laid-Open No. 2000-248213 discloses a recording liquid containing a polyurethane polymer in which a polyisocyanate has a symmetrical six-membered ring.
The present inventors found that conventional inks containing a pigment and a polyurethane polymer have improved ink reliability and image fastness property but still do not satisfy recent requirements in business.
The present inventors found that in polyurethane polymers as described in Japanese Patent Laid-Open No. 2008-266595, a decrease in acid value increases hydrophobicity, resulting in a marked decrease in ink ejection stability. On the other hand, an increase in acid value improves ink ejection stability but disadvantageously increases the proportion of a hard segment in the polyurethane polymer, resulting in a decrease in the scratch resistance and highlighter resistance of images. The hard segment is mainly composed of a polyisocyanate, a compound having an acid group, and a chain extension agent. Thus, it is difficult even for an ink according to Japanese Patent Laid-Open No. 2008-266595 to satisfy both ink ejection stability and the scratch resistance and highlighter resistance of images.
In Japanese Patent Laid-Open No. 2000-248213, the polyurethane polymer has only a sulfo group as an acid group to improve the dispersion stability of carbon black. However, there is no discussion on the effects of the polyurethane polymer having a sulfo group on ink ejection stability or the scratch resistance and highlighter resistance of images. Furthermore, the polyurethane polymer having a sulfo group alone has an insufficient hydrophilic-hydrophobic balance, resulting in low scratch resistance and highlighter resistance of images.